Titanium and titanium-alloys are important metals for several applications. Titanium-alloys are used for aircraft and missiles where lightweight strength and high temperature performance are important. Titanium-alloys generally include a stabilizing element that alters the transformation temperature of specific metal phases in titanium to alter the material characteristics of the titanium.
Titanium and some titanium-alloys can be heat treated to increase the strength of the material. However, when titanium is exposed to high temperatures in the presence of oxygen, a hard brittle layer caused by oxygen diffusing into the titanium, called an alpha case, is formed. Alpha case is a definite drawback to titanium usage as it can affect fatigue strength, corrosion resistance, and limits titanium's high temperature capability with respect to mechanical properties. For example, while titanium is an incredibly strong metal, an alpha case layer can reduce the amount of strain that the surface can withstand before cracking. If the metal cracks it creates a stress concentration that could result in fatigue crack propagation, potentially leading to a catastrophic part failure.
Alpha case forms in a layer and does not affect the properties of the interior titanium-alloy. However, if left on a part, alpha case can cause the part to fail in applications where it would not fail if no alpha casing was present.
There are at least three ways to deal with alpha case formation on titanium: prevention, minimization, or removal. Prevention can include not exposing the titanium to high temperature, and not exposing the titanium to high temperature in the presence of oxygen. As heat treating titanium provides an advantageous increase in strength, foregoing heat treating titanium is often not an option.
Heat treating in an oxygen depleted environment is another possible solution, but it adds significant costs. In addition, it is difficult to remove 100% of the oxygen from the atmosphere, particularly in a production situation. Heat treating in an oxygen depleted environment is often more an exercise of minimizing the formation of alpha case, rather than preventing the formation of alpha case. For example, U.S. Pat. No. 6,814,818 to Woodfield et al. discloses a method of heat treating titanium-alloy articles in a vacuum furnace to limit formation of alpha case. The disclosed process requires the identification of an acceptable alpha case thickness and then seeks to limit the thickness of the alpha case by limiting the availability of oxygen during heat treating.
Removal of the alpha case is the third solution. Prior art alpha case removal methods include mechanical or chemical milling where a part is manufactured oversized, heat-treated, and then the resultant alpha case is removed from the oversized part, leaving a finished part, smaller than the oversized manufactured part initially produced.
Chemical milling consists of forged products being dipped into vessels filled with strong acids, hydrofluoric or nitric, to remove the alpha case. This process is not an ideal solution to the problem because regulations in the industry cause chemical milling to be an expensive process to maintain. It also puts the manufacturer at risk in the unlikely situation where an undesirable chemical exposure may occur. Furthermore, the spent hydrofluoric acid has to be disposed of, causing further disposal concerns. Finally, it can be difficult to ensure uniform material removal, making it difficult to control a process that removes a consistent amount of material while minimizing the amount of material removed, acid spent and process time while consistently ensuring removal of the entire alpha case layer.
Prior art mechanical milling also requires manufacture of an oversized part that is heat treated and then processed again through a mechanical milling material removal process such as cutting, grinding, abrasive blasting, electrical discharge machining, electron beam machining and ultrasonic machining to remove the alpha case layer. Such mechanical milling can be difficult to successfully implement in production because the alpha case layer is present on all surfaces, so mechanical milling has to remove a balanced quantity of material from all surfaces. For example, when refinishing the top and bottom surface on a part, it is critical that a first mechanical milling operation on the top surface removes the correct amount of material, because if too much material is removed, a subsequent mechanical milling operation on the bottom surface may leave residual alpha case material or may reduce the distance between the top and bottom surfaces below a tolerance range. Mechanical milling is also undesirable because it involves an additional pass through machining centers, significantly increasing the manufacturing costs for a part.
Furthermore, all of the prior art solutions can require quality control be performed on the finished product to ensure that no more than the acceptable amount of alpha case remains on the part. This can add additional costs to the final manufacturing cost of titanium parts.
Accordingly there is a need for an improved method to deal with alpha case on manufactured titanium parts.